A green roof is one kind of sustainable roof design that incorporates landscape features as an integral part of the roof system.  The two main types are known as an extensive roof system and an intensive roof system.  Both of these vegetative roof types can provide substantial economic and environmental advantages over conventional roof construction.

An extensive roof system is a vegetative roof system which is not habitable.  It can be installed on most roofs with little or no slope.  Due to natural irrigation measures, an extensive roof system requires little or no maintenance and it includes plant life such
as mosses and grasses on a shallow soil bed.

The second type of vegetative roof system is known as an intensive roof system.  It differs from the extensive kind in that it is usually intended for human use.  In addition, the plant life can be elaborate and include almost any type of climate appropriate vegetation including mature trees or shrubs.  Due to human access and mature plant life, the structural implications of an intensive roof system are much greater than that of an extensive roof system.  However, the increased usable space on a given property may justify the cost of additional building structure considerations for increased soil depths and live loads.

Although the initial cost of a green roof may be twice that of a conventional roof, there are several advantages that both an extensive and intensive roof can provide.  In terms of materials, green roofs can last three to four times longer than typical roof construction. 

This is achieved by protection of the roof membrane system from direct sunlight with soil and vegetation.  These elements also help reduce storm water runoff and improve air quality. 

In addition, green roof construction affords increased insulation against sound and thermal transmission.  The latter decreases the required heating and cooling loads of mechanical systems and can significantly reduce energy costs.

http://www.greenroofs.org/
http://www.greenroofs.com/